In a prior art method of this kind (Siemens KMT-Report, January 1996, pages 10 and 11) a mobile central unit as an auxiliary device is connected to a special line in the form of a communications transmission link which is firmly connected at at least one end thereof. In practice, said auxiliary device is connected adjacent to one end of the communications transmission link. In this prior art method, a central measuring system (measuring system MSy90) is connected to said special line. The auxiliary device is used for measuring quality determining parameters of the communications transmission link.